Wireless communication systems are widely deployed to provide, for example, a broad range of voice and data-related services. Typical wireless communication systems consist of multiple-access communication networks that allow users to share common network resources. Examples of these networks are time division multiple access (“TDMA”) systems, code division multiple access (“CDMA”) systems, single-carrier frequency division multiple access (“SC-FDMA”) systems, orthogonal frequency division multiple access (“OFDMA”) systems, or other like systems. An OFDMA system is adopted by various technology standards such as evolved universal terrestrial radio access (“E-UTRA”), Wi-Fi, worldwide interoperability for microwave access (“WiMAX”), ultra mobile broadband (“UMB”), and other similar systems. Further, the implementations of these systems are described by specifications developed by various standards bodies such as the third generation partnership project (“3GPP”) and 3GPP2.
As wireless communication systems evolve, more advanced network equipment is introduced that provide improved features, functionality, and performance. A representation of such advanced network equipment may also be referred to as long-term evolution (“LTE”) equipment or long-term evolution advanced (“LTE-A”) equipment. LTE is the next step in the evolution of high-speed packet access (“HSPA”) with higher average and peak data throughput rates, lower latency and a better user experience especially in high-demand urban areas. LTE accomplishes this higher performance with the use of broader spectrum bandwidth, OFDMA and SC-FDMA air interfaces, and advanced antenna methods.
Communications between wireless devices and base stations may be established using single-input, single-output (“SISO”) mode, where only one antenna is used for both the receiver and transmitter; single-input, multiple-output (“SIMO”) mode, where multiple antennas may be used at the receiver and only one antenna is used at the transmitter; multiple-input, single-output (“MISO”) mode, where multiple antennas may be used at the transmitter and only one antenna is used at the receiver; and multiple-input, multiple-output (“MIMO”) mode, where multiple antennas may be used at the receiver and transmitter. Compared to SISO mode, SIMO mode may provide increased coverage while MIMO mode may provide increased coverage and spectral efficiency and higher data throughput if the multiple transmit antennas, multiple receive antennas or both are utilized. When wireless devices using MIMO mode are employed additional MIMO operating modes are available. These operating modes include diversity MIMO mode, single-user MIMO mode, multiple-user MIMO mode and mixed MIMO mode. Diversity MIMO-mode uses multiple transmit and receive antennas to take advantage of the spatial dimensionality of the wireless communication radio frequency (“RF”) channel to provide more reliable transmission of a single data channel. It is important to recognize that systems employing base stations using MIMO mode can typically support wireless devices operating in SISO mode, SIMO mode, MISO mode, MIMO mode, other operating modes or combinations of operating modes.
Single-user MIMO (“SU-MIMO”) mode takes advantage of the spatial dimensionality of the wireless communication RF channel by using multiple transmit and receive antennas to provide multiple concurrent transmission data channels for increased data rates of a single wireless device. Similarly, multiple-user MIMO (“MU-MIMO”) mode uses multiple transmit and receive antennas to provide multiple concurrent transmission data channels to multiple wireless devices. Mixed MIMO mode concurrently supports the combination of SIMO and MIMO wireless devices on the same RF channel. Uplink (“UL”) communication refers to communication from a wireless device to a base station. Downlink (“DL”) communication refers to communication from a base station to a wireless device.
As specified in 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Channels and Modulation (Release 8), 3GPP, 3GPP TS 36 series of specifications (“LTE Release 8”), the use of multiple antenna techniques is supported for DL transmission. In 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Further Advancements For E-UTRA; Physical Layer Aspects (Release 9), 3GPP, 3GPP TR 36.814 V1.1.1 (2009 June) (“LTE-A Release 10”), multiple antenna techniques may be used to improve DL performance. Such multiple antenna techniques include, for instance, transmit diversity and spatial multiplexing. Various transmit diversity schemes may be used such as space-frequency block coding (“SFBC”), space-time block coding (“STBC”), frequency-switched transmit diversity (“FSTD”), time-switched transmit diversity (“TSTD”), pre-coding vector switching (“PVS”), cyclic-delay diversity (“CDD”), space-code transmit diversity (“SCTD”), spatial orthogonal resource transmission diversity (“SORTD”), and other similar approaches. Some of these approaches have been adopted for use in LTE Release 8.
Channel estimation techniques including for use in orthogonal frequency division multiplex (“OFDM”) systems have been extensively described in the literature. Coleri et al., A Study of Channel Estimation in OFDM Systems, IEEE proc. VTC 2002 Fall, pp. 894-898, presented an overview of channel estimation and interpolation techniques for OFDM systems. Simplified minimum mean-square error (“MMSE”) estimators are described in Edfors et al., OFDM Channel Estimation by Singular Value Decomposition, IEEE Trans. On Commn., Vol. 46, Issue 7, July 1998, pp. 931-939, and Van de Beek et al., On Channel Estimation in OFDM Systems, IEEE VTC, 1995, pp. 815-819. Hsieh et al., Channel Estimation for OFDM Systems Based on COMP-type Pilot Arrangement in Frequency Selective Fading Channels, IEEE Trans. on Consumer Electronics, Vol. 44, No. 1, February 1998, pp. 217-225, describes performance results comparing MMSE estimation with transform-domain interpolation. Hadaschik et al., Joint Narrowband Interference Detection and Channel Estimation for Wideband OFDM, Proceedings of European Wireless Conference, April 2007, explores the narrow-band interference detection and channel estimation in wideband OFDM. In most of these publications, the channel estimation techniques do not consider the effects of co-channel inter-cell interference. In a typical network deployment, inter-cell interference can affect the channel estimation calculated at a wireless device.
In LTE systems, a reference signal (“RS”) is a pre-determined signal, typically known by both base stations and wireless devices and used for channel estimation. The RS may also be referred to as a pilot signal, training signal, synchronization signal, sounding signal or other similar term. A base station typically transmits a plurality of RS signals to allow each wireless device to estimate the propagation properties of the RF channel. Upon deriving channel estimates, such estimates may also be used for demodulation of the information transmitted by a base station.
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